1. Field of the Invention
This invention relates to computer memories and, more particularly, to ferroelectric memories.
2. Discussion
Semiconductor memories are used in computer systems to perform a variety of functions--bulk data storage, program storage, temporary storage, and cache (or intermediate) storage. Semiconductor memories fall into two general categories: nonvolatile and volatile memories. Nonvolatile memories are randomly accessible memories which do not lose information loaded into their storage cells even if electrical power is shut off. Nonvolatile memories include mask programmable read only memories (ROMs), fuse programmable read only memories (PROMs), ultraviolet erasable programmable read only memories (UVEPROMs), electrically alterable read only memories (EAROMs), electrically erasable programmable read only memories (EEPROMs), and nonvolatile static RAMs (NV RAMs).
Volatile memories, on the other hand, typically will lose information stored in their storage cells when power to the memory circuit is shut off. The most common type of volatile memory is the read/write random access memory (RAM). This memory has the distinct advantage that data can be written into the memory as well as read out of it. There are two different types of read/write RAMs--dynamic and static. Dynamic RAMs use a storage cell based on a transistor and capacitor combination and will lose their charge unless the charge is repeatedly replenished (refreshed) on a regular basis (every few milliseconds). If refreshed, the information will remain until intentionally changed or the power to the memory is shut off. Static memories, in contrast, do not use a charge--storage technique; instead, they use either four or six transistors to form a flip-flop for each cell in the array. Once data is loaded into the flip-flop storage elements, the flip-flop will indefinitely remain in that state until that information is intentionally changed or the power to the memory circuit is shut off. To avoid the loss of memory in volatile memories, such as dynamic and static RAMs during a power loss, most computer systems utilize some sort of battery back-up to insure continued power to the volatile memories. An additional problem with dynamic and static RAMs is the destruction of the data in the memory when their memory is being read. Thus, it would be desirable to provide a read/write RAM memory which is non-volatile. It would further be desirable to provide such a memory with a nondestructive readout.
Ferroelectric capacitors have recently been employed in an effort to develop a nonvolatile RAM. Ferroelectrics are crystalline substances which have a permanent spontaneous electric polarization that can be reversed by an electric field. Consequently, thin-film ferroelectric capacitors, which can be permanently polarized to store digital data, are being substituted for the silicon dioxide capacitors typically used in the standard RAM memory cell to store charge and hence data. See F. P. Gnadinger and D. W. Bondurant, "Ferroelectrics for Nonvolatile RAM's", IEEE Spectrum, July, 1989, page 30, for a review of current research in this area. Besides being nonvolatile, ferroelectric RAMs may yield additional advantages. These include a higher charge density which means a smaller capacitor permitting smaller overall memory size. Also, higher switching speeds, longer endurance (the number of read/write cycles a memory can undergo before losing the ability to store data), and better data retention due to reduced charge leakage are possible.
An additional advantage is that ferroelectric memories are inherently radiation hard. On the other hand, in ferroelectric memories, the readout process typically destroys the data, thus requiring an immediate rewrite to restore it. While performing the rewrite is not difficult, it would be desirable to have a ferroelectric memory with nondestructive readout so that the rewrite may be avoided. Further, if the rewrite cycle is disrupted, for example, if the circuitry is exposed to transient radiation during the rewrite, the rewrite may fail with consequent loss of data. Therefore, for operation in such environments, it would especially be desirable to have a memory with non-destructive readout, so that rewriting is unnecessary.